Typically, PLCs used for factory automation (FA) or other purpose create data backup in the event of a power cut.
For example, a power supply switching unit included in a PLC described in Japanese Unexamined Patent Application Publication No. 2014-160377 (Patent Literature 1) includes two power supply lines that are both connected in parallel to a direct current (DC) power supply. A DC-DC converter is connected to the first power supply line via a diode. This DC-DC converter lowers an input voltage (20 to 28 V) to 3.3 V, which is an operating voltage of a main circuit, and outputs this voltage as a constant voltage.
A capacitor, which serves as a power supply for a backup operation, is arranged between the second power supply line and a ground. The capacitor is connected to an input stage of the DC-DC converter via a switch. While the DC power supply is on, the above switch is off and the capacitor is disconnected from the DC-DC converter. When a power cut is detected, the switch is turned on to cause the DC-DC converter to generate a constant voltage for a predetermined period using the capacitor as its power supply.